1. Field of the Invention
This invention relates generally to a hermetic feedthrough terminal pin assembly, preferably of the type incorporating a filter capacitor. More specifically, this invention relates to metallization comprising oxidized titanium for incorporation into feedthrough filter capacitor assemblies, particularly of the type used in implantable medical devices such as cardiac pacemakers, cardioverter defibrillators, and the like, to decouple and shield internal electronic components of the medical device from undesirable electromagnetic interference (EMI) signals. The metallization provides a surface with which a hermetic seal can be established that prevents passage or leakage of fluids through the feedthrough assembly and into the medical device.
2. Prior Art
Feedthrough assemblies are generally well known in the art for use in connecting electrical signals through the housing or case of an electronic instrument. For example, in an implantable medical device, such as a cardiac pacemaker, defibrillator, or neurostimulator, the feedthrough assembly comprises one or more conductive terminal pins supported by an insulator structure for passage of electrical signals from the exterior to the interior of the medical device. The conductive terminals are fixed into place using a metallization and gold braze process, which provides a hermetic seal between the pin and insulative material.
Conventionally, a metallization is composed of a combination of discrete layers of untreated titanium metal and molybdenum or titanium metal and niobium have been used to facilitate bonding of the gold to the insulative material. Untreated titanium metal is widely used as an adhesion layer to provide bonding between a ceramic material, particularly that of alumina, and a different metal. However, the untreated titanium metal typically reacts with gold to form an intermetallic alloy. Intermetallic alloy metals such as those formed by the combination of titanium and gold, typically result in an undesirable brittle bond which may result in loss of hermeticity. Titanium metal is known to have a high diffusion coefficient in liquid gold which increases its tendency to diffuse within gold and form these intermetallic alloy phases. Typically when such metals are brazed, the titanium metal departs or lifts from the surface of the insulator material and forms an intermetallic alloy with the gold braze material.
As a result, a barrier layer comprising molybdenum or niobium is applied to the outer surface of the titanium. This additional layer is designed to act as a barrier layer to prohibit the migration of titanium from the surface of the insulator material and thus prevent the formation of a titanium and gold intermetallic. While materials such as molybdenum and niobium typically provide adequate metallization barrier layers, recent work has been focused on an improved metallization layer through incorporation of an oxidized layer of titanium as a means to facilitate bonding of ceramic with that of a metal with minimized migration of the metallization layer. The diffusion rate for the oxidized titanium in gold is less than that of the untreated titanium. Therefore, the metallization comprising the oxidized metal is less likely to lift from the surface of the insulator and form an intermetallic phase with the gold braze material.